In a photostimulable phosphor imaging system, as described in U.S. Pat. No. Re. 31,847 reissued Mar. 12, 1985 to Luckey, a photostimulable phosphor sheet is exposed to an imagewise pattern of short wavelength radiation, such as x-radiation, to record a latent image pattern in the photostimulable phosphor sheet. The latent image is read out, by stimulating the phosphor with a relatively long wavelength stimulating radiation, such as red or infrared light. Upon stimulation, the photostimulable phosphor releases emitted radiation of an intermediate wavelength, such as blue or violet light, in proportion to the quantity of short wavelength radiation that was received. To produce a signal useful in electronic image processing, the photostimulable phosphor sheet is scanned in a raster pattern by a beam of light produced for example by a laser deflected by an oscillating or rotating scanning mirror, and the emitted radiation is sensed by a photodetector such as a photomultiplier tube to produce the electronic image signal.
In one type of scanning apparatus, the photostimulable phosphor sheet is placed on a translation stage, and is translated in a page scan direction past a laser beam that is repeatedly deflected in a line scan direction to form the scanning raster.
To optimize the signal-to-noise ratio (S/N) of the imaging system, it is desirable to collect as much of the emitted light as possible and to direct it to the photodetector. While the apparatus employed to collect the light may take various forms, one form of light collector is proposed in U.S. Pat. No. 4,616,129, issued Oct. 7, 1986, to Yamada et al. The light collector proposed by Yamada et al comprises a slab of light transmitting material 20-30 mm thick. One face of the slab is positioned adjacent the scan line to collect emitted light. An array of rectangular faced photomultiplier tubes are arranged along an opposite face to receive the collected light.
Experiments have identified a factor that limits the signal-to-noise ratio achievable with the photostimulable phosphor imaging apparatus. As the photostimulable phosphor sheet is scanned by the stimulating radiation beam, a high percentage (up to 90%) of the stimulating radiation is reflected from the photostimulable phosphor. When this reflected stimulating radiation is further reflected back on to the surface of the photostimulable phosphor, in a location away from the instantaneous scanning point, it is called flare and the phosphor will be stimulated to emit in these other locations. Flare induced emission which is collected by the light collector gives rise to a spurious background signal. Such reflection of the stimulating radiation onto the photostimulable phosphor may occur from the light collecting face of the light guide described above. Examples of the image degradation caused by flare include a reduction in the contrast of images due to flare induced emission from high exposure areas, which adds unwanted signal to low exposure areas. Shadow artifacts are produced in the image when a high exposure object on a low exposure background field is scanned. The signal-to-noise radio in all image areas is degraded. Lase noise is enhanced since a large area of the phosphor is exposed to a low level of stimulating radiation, the light emitted from this area will follow the fluctuations in laser power, thereby amplifying the effect of the laser noise.
It is therefore the object of the present invention to provide an improved light collector having reduced flare.